FIG. 1 shows a conventional scanning electron microscope capable of automated focusing. This microscope has an electron gun (not shown) producing an electron beam 1 which is accelerated. The microscope is further equipped with two stages of deflection coils 2 and 3 each of which contains horizontal and vertical deflection coils. The beam is sharply focused by an objective lens 4 and made to hit a specimen 5. The resulting electrons such as secondary electrons are detected by a detector 6.
A horizontal scanning signal-generating circuit 7 supplies horizontal scanning signals to the horizontal deflection coils of the two stages of deflection coils 2 and 3 via a driver circuit 8. A vertical scanning signal generating circuit 9 supplies vertical scanning signals to the vertical deflection coils of the two stages of deflection coils via a driver circuit 10. The frequencies of these scanning signals are controlled by a control circuit 11 consisting of a computer.
The output signal from the detector 6 is fed via an amplifier 12 to a CRT 13 and a filter circuit 14, the CRT 13 being supplied with the horizontal and vertical scanning signals. The output signal from the filter circuit 14 is furnished via an absolute value circuit 15 to an integrator circuit 16, which integrates its input value. The output value from the integrator circuit 16 is converted into digital form by an A/D converter 17. The output signal from the A/D converter 17 is stored in a signal intensity distribution memory 19 included in the control circuit 11.
The control circuit 11 further includes a maximum value detector unit 20 for detecting the maximum value of numerous integrated values stored in the signal intensity distribution memory 19. The control circuit 11 is further equipped with an objective lens value setting data memory 21 and an auxiliary coil value setting data memory 22. The value of the objective lens value setting data memory 21 is supplied to an objective lens driver circuit 24 via a D/A converter 23. The value of the auxiliary coil value setting data memory 22 is supplied via a D/A converter 25 to a driver circuit 27 which acts to drive an auxiliary coil 26. The microscope further includes an objective lens value conversion unit 28 and an autofocus unit 29.
The electron microscope constructed as described above operates in the manner described now. When a secondary electron image is observed, the horizontal scanning signal-generating circuit 7 and the vertical scanning signal-generating circuit 9 which are under the control of the control circuit 11 supply scanning signals to the deflection coils 2 and 3, the scanning signal giving a desired scanning speed. The electron beam 1 is deflected by the deflection coils 2 and 3 and scans a desired region on the specimen 5. The irradiation of the electron beam upon the specimen 5 generates secondary electrons, which are detected by the detector 6. The output signal from the detector 6 is amplified by the amplifier 12 and fed to the CRT 13 that is supplied with the scanning signal from horizontal and vertical scanning signal generators 7 and 9, respectively. As a result, a secondary electron image of the specimen is displayed on the CRT 13.
An automated focusing operation is described now by referring to the operating characteristic curve of the auxiliary coil 26 shown in FIG. 2(a) and to the operating characteristic curve of the objective lens 4 shown in FIG. 2(b). In these two graphs, the intensity of excitation which corresponds to the distance between the specimen surface and the lens is plotted on the vertical axis. Time is plotted on the horizontal axis. First, an initial value .DELTA.Z shown in FIG. 2(b) is set into the objective lens value setting data memory 21. The objective lens driver circuit 24 is operated according to this initial value and excites the objective lens 4. Then, the autofocus unit 29 begins to operate and the value of the auxiliary coil value setting data memory 22 is varied linearly as shown in FIG. 2(a). More precisely, the set value is varied in a stepwise fashion as shown in FIG. 3(a).
Whenever the set value is varied in an increment in this way, a scanning signal is supplied to the deflection coils 2 and 3 so that the desired region on the specimen 5 may be scanned once. The resulting secondary electrons are detected by the detector 6. The output signal from the detector 6 is amplified by the amplifier 12. Components of the output signal from the amplifier 2 which are in a certain range are filtered out by the filter circuit 14. Every negative signal component is inverted by the absolute value circuit 15.
The output from the absolute value circuit 15 is supplied to the integrator circuit 16, which integrates its input signal. This integration operation is carried out during one two-dimensional scan of the desired region on the specimen. After the end of this scanning operation, the integrated value is stored in the signal intensity distribution memory 19 included in the control circuit 11 via the A/D converter 17. This integration operation is effected whenever the excitation of the auxiliary coil 26 is varied in an increment. When the excitation from +A to -A is completed, the distribution shown in FIG. 3(b) is stored in the memory 19.
The maximum value-detecting unit 20 in the control circuit 11 detects the maximum value of the distribution shown in FIG. 3(b). The corresponding intensity of excitation of the auxiliary coil 26 is supplied to the objective lens value conversion unit 28. When the excitation is maximal in this manner, the electron beam is in focus. As a result, the value of the objective lens value setting data memory 21 is the initially set value .DELTA.Z plus the intensity of excitation .DELTA.Z1 (i.e., the deviation of the value at the focal point from the initial value .DELTA.Z) of the auxiliary coil when the beam is in focus. After the excitation of the objective lens 4 is set to this value .DELTA.Z+.DELTA.Z1, the aforementioned automated focusing operation is conducted again. At this time, the amount of deviation .DELTA.Z2 of .DELTA.Z+.DELTA.Z1 from the value at the focal point is stored in the auxiliary coil value setting data memory 22. The excitation value .DELTA.Z1 of the objective lens and the excitation value .DELTA.Z2 of the auxiliary coil are shown in FIGS. 2(a) and 2(b). When these excitation values .DELTA.Z1 and .DELTA.Z2 are stored in the data memory, the automated focusing operation is ended.
In the method described above, an automated focusing operation is automatically started by setting the autofocus unit 29 into operation. Therefore, when the stage carrying a specimen is moved or in other similar situations, the beam comes out of focus. This makes it impossible to observe an image which is kept in focus. Also, when the automated focusing operation is started, the excitation of the auxiliary coil 26 varies from moment to moment, thus changing the state of the image. Hence, the image cannot be observed well.
It is an object of the present invention to provide a scanning electron microscope for creating an image that is kept in focus and stable even if an automated focusing operation is being performed.